Layover, counted in hours you can actually use
A layover is the time between two flights at Schiphol, but the usable part is what is left after the terminal, the border and the return trip.
An itinerary counts a layover from scheduled landing to scheduled departure. For a session, what matters is the smaller number inside it: the hours you are actually in a room.
What comes off first
Taxiing to the stand, leaving the aircraft, the walk along the pier, border control if you are entering Schengen, the baggage belt if your case is not tagged through, and the ride to the hotel. On the way back, the ride, security, outbound border control if you are leaving Schengen, and the margin the airline wants before the gate closes. On a six hour layover these often add up to three hours or more.
What fits in what is left
Under four scheduled hours, a session rarely fits. Between four and six, one hour at EUR 180 usually does. Between six and ten, ninety minutes or two hours fits with time to shower and dress without hurry. A layover that crosses a night is a different kind of booking, closer to a hotel stay than a stop. The page on massage between flights sets out the subtraction in full.
Whether you can leave at all
Some passengers cannot leave the transit area on a given ticket or passport, and for them a session on that layover is not possible. The transit area entry explains the distinction, and it is worth checking before anything else.